A belt CVT (Continuously Variable Transmission) is known as a change gear installed in automobiles. A belt CVT comprises a primary pulley for inputting the driving force of the engine, a secondary pulley for outputting the driving force to the driving wheel, and a V-belt wound around the primary pulley and secondary pulley for transmitting the driving force inputted from the primary pulley to the secondary pulley. The pulley width of the primary pulley and the secondary pulley can be varied by the oil pressure supplied to the primary pulley and secondary pulley. When changing gears, oil pressure supplied to the primary pulley and secondary pulley adjusts the pulley width and thus to vary the ratio (pulley ratio) of the contact radius (effective radius) in relation to the primary pulley and secondary pulley of the V-belt. Thus, the ratio of rotation speed (gear ratio) between input and output is adjusted.
It is important that belt slippage does not occur in the belt CVT in order to prevent reduction in belt durability. Belt slippage can be prevented by increasing the oil pressure supplied to the primary pulley and secondary pulley to thereby increase the force at which the primary pulley and secondary pulley hold the V-belt.
In cold climates, it is not uncommon for the temperature of the oil supplied to the primary pulley and secondary pulley to fall below −20° C. while the automobile is parked. Under such conditions, since the pressure of the oil supplied to the primary pulley and secondary pulley does not rise due to the increased oil viscosity, it is possible that the force for holding the V-belt between the two pulleys will be insufficient and that belt slippage will occur. In such cases, the prior art prevents the torque from increasing by limiting the engine output torque, which results in prevention of belt slippage.